Roof Windows and Trimmer Rafters

I’ve just had two 78cm by 98cm Velux windows fitted by a local roofer into the rear slope of my 1920’s terraced house roof. The windows are placed on the same level in the slope with just over 800mm of roof surface between them. The rafters are 4inch by 2inch on approximately 400 mm centres and extend around 7ft from purlin to ridge (apologies for mixed units).

The size of the windows has necessarily required that two rafters be cut for each making a total of four rafters cut. In framing the two openings the roofer has fitted double 4x2 trimmer timbers to form the header and footer of each window but has not doubled up on the trimmer rafters each side of the openings. I was under the impression this was good practice when several (four in this case) rafters have been cut and also I would have thought this was at least if not more important than doubling up the trimmers.

However the thing that really started bothering me was the fact that the roofer has chosen to cut rafters in such a way that there are now only two continuous uncut rafters in the portion of roof between the two openings and both of these are being called on to be trimmer rafters. My instincts tell me that these two rafters have become almost like critical components and must be backed up. I have requested that the roofer double up these trimmer rafters. What do other members think?

The infuriating thing is that the situation I describe could have been easily avoided. To make it clear what I mean, imagine the original rafters being numbered from left to right. The roofer has cut rafters 6 and 7 for the first opening so that rafters 5 and 8 then become the trimmer rafters for the first opening – the window sits to the right hand side in this opening. The roofer has then cut rafters 10 and 11 so that rafters 9 and 12 become the trimmer rafters for the second opening – the window sits to the right in this opening. To my mind, for this second opening it would have been much better to have cut rafters 11 and 12 so that rafters 10 and 13 become the trimmer rafters and to fit the window to the left hand side in this opening (the window would have been just a few mm to the right of its current position). That way there would have been three intact rafters (8, 9 and 10) between the windows instead of just two (8 and 9).

My mind began to wander about half way through the third paragraph!

A photo speaks a thousand words or in your case 2000 words!

Best photo I have, I’m afraid. Still you can see the two cut rafters for the right hand window opening. The adjacent left hand opening is just visible. In the roof area between these two openings there are just two intact rafters both of which will be called on to be the trimmer rafters. It is these two rafters I have requested be doubled up.

Here’s a better photo showing the roof area between the two windows. You can see there are only two uncut rafters remaining and both of these are used as trimmer rafters.

I’m not a great fan of rules of thumb but isn’t there a “good practice” rule that says “cut 1 or 2, add 2” (I’ve seen it in Goss’s Roofing Ready Reckoner). This would mean in my situation that the trimming rafters at both sides of both window openings should be doubled up and I would have thought this to be especially advisable in the area between the window openings where there are no other rafters nearby to act as a backup.

Your rule of thumb; "cut 1 or 2, add 2" is correct. That'd be an extra one either side. Similarly "cut 3 or 4, add 4, that's 2 either side."
Whether it's good practice or regulations, I don't know, but it's a rule I've always followed.
The additional rafters may not need to go the full span, just between purlins.

The distance between the rafters in the roof construction usually decides the size of the windows if cutting them and install- ing trimmers is not an option. Therefore, first check whether the position chosen and size of the roof windows fit into the existing roof construction or if rafters will need cutting (see pages 16, 48).

In other words double the rafters